RE: emacs-25 f708cb2: Clarify doc string of 'transpose-sexps'

[Drew Adams <drew.adams@oracle.com>]

> > It is really not great to tell someone they "cannot" or
> > "must not" etc. do something without raising an error that
> > really enforces "cannot" etc.
> 
> If someone comes up with a way to lift that restriction, we could
> remove this text from the doc string.  but as long as the command
> works as it does, I think we should document that, or else we will
> have bug reports about its not meeting expectations.

I did not suggest removing the text from the doc string.

The point is that it is better if the code backs up what
the doc says, by raising an error if you try to do what
it says you cannot do.

If `C-b' at bob did not raise an error, but instead just
moved point to the end of line 42, it would be scant help
to tell users in the doc of `C-b' that you cannot use it
at bob.